(a) Field of the Invention
The present invention relates to carburetor constructions, and more particularly to an improvement of a slide and pin type carburetor in which a slide valve for controlling the flow rate of suction air flowing through a main intake passageway by moving in a direction transverse to the main intake passageway comprises; a flat plate portion located on the upstream side of a main fuel nozzle opened to the main intake passageway and sliding along guide grooves formed on opposite sides of the main intake passageway; and a column-shaped portion, formed integral with the flat plate portion on the downstream side thereof, having a fuel metering needle fitted into the main fuel nozzle to form an annular fuel passageway.
(b) Description of the prior art:
The carburetor of the type, which has been known in the past, is shown in FIGS. 1 and 2 as an example. In FIGS. 1 and 2, reference numeral 1 denotes a carburetor body having a main intake passageway 3, through which suction air flows in the direction indicated by an arrow 3. Reference numeral 4 denotes a slide valve, which comprises guide grooves 5 formed on opposite sides of the main intake passageway 2, a flat plate portion 6 fitted into the guide grooves 5 to slide along the grooves, and a column-shaped portion 7 formed integral with the flat plate portion 6 on the downstream side thereof, on the bottom of which a fuel metering needle or pin 8 is mounted. The fuel metering needle 8 is fitted into a main fuel nozzle (not shown) opened to the main intake passageway 2 and formed an annular fuel passageway. This slide valve type carburetor controls the flow rate of air in accordance with the extent of the advance of the flat plate portion 6 of the plate-shaped slide valve 4 into the main intake passageway 2, so that, when the valve is fully closed, that is, in a state that the main intake passageway 2 is completely blocked by the flat plate portion 6, a negative pressure or a vacuum acting for the exit of the main fuel nozzle will become too high followed by the suction of an excessive amount of fuel and it has been difficult to control the flow rate of fuel through the annular fuel passageway formed by the main fuel nozzle and the fuel metering needle 8. The use of the carburetor having the slide valve of such a structure, therefore, has been limited to a particular field such as racing cars.
FIG. 3 shows the plate-shaped slide valve 4 forming a cutaway 9 at the center of the lower end of the flat plate portion 6 of the above-mentioned plate-shaped slide valve 4 and FIGS. 4 and 5 are structural views of the slide and pin type carburetor disclosed in U.S. Pat. No. 4,013,741, in which the plate-shaped slide valve 4 shown in FIG. 3 is used. That is to say, in FIGS. 4 and 5, reference number 1 represents a carburetor body, 2 represents a main intake passageway through which air flows in the direction of an arrow 3, and 4 represents a plate-shaped slide valve which comprises a flat plate portion 6 sliding along guide grooves 5 and a column-shaped portion 7, in which a cutaway 9 is formed at the center of the lower end of the flat plate portion 6 and a fuel metering needle 8 is mounted in the column-shaped portion 7 and is fitted into a main fuel nozzle 10 opened to the main intake passageway 2 to form an annular fuel passageway. These aspects are the same as in the slide and pin type carburetor shown in FIG. 2. The provision of the cutaway 9 can prevents an abnormally high vacuum from being produced around the exit of the main fuel nozzle 10 at a fully closed position of the plate-shaped slide valve 4. However, there is no difference between them in that the control of the flow rate of suction air is performed on the upstream side of the main fuel nozzle 10, and a static negative pressure is largely produced, due to a choke action, around the exit of the main fuel nozzle 10, so that the negative pressure brought about around the exit of the main fuel nozzle 10 cannot freely be set even through the cutaway 9 is provided.